The present invention relates to an image detecting apparatus and method used to observe and measure the shape of microstructures or three-dimensional structures of specimens.
Conventionally, confocal microscopes have been used as image detecting apparatus. A typical confocal microscope is one that uses a disk having a large number of pinholes arranged in the form of spirals, i.e., a Nipkow disk. In such a confocal microscope, in order to produce a confocal image of a specimen, it is scanned with a beam of light by rotating the Nipkow disk. If, when the confocal image of the specimen is captured by a CCD camera or the like, scanning of the specimen by the disk and image capture by the camera are not properly synchronized with each other, then light and dark bars may be produced on captured images.
To solve such a problem, a method has been proposed which synchronizes the disk rotation with the CCD camera using a video signal from the camera as disclosed in Japanese Unexamined Patent Publication No. 9-297267.
Most of the conventional apparatuses use a brushless DC motor that is comparatively inexpensive and easy to use. Usually, the speed control of such a brushless DC motor is performed by controlling the magnitude of a motor drive signal input to a motor driver and thereby changing the magnitude of current in the motor stator coil. As the magnitude of the drive signal increase, the motor speed increases with increasing motor stator coil current and vice versa.
As an invention regarding control of motor stator coil current for motor speed control, there is a motor control device as disclosed in Japanese Unexamined Patent Publication No. 7-250492.
FIG. 1 shows the arrangement of such a device. A brushless DC motor (hereinafter referred to simply as a motor) 111 is provided with delta-connected stator coils 112a, 112b and 112c and a rotor 113 formed with four magnetic poles. Hole elements 114a, 114b and 114c are placed opposite to the motor stator coils 112a, 112b and 112c, respectively. Hall signals E1, E2 and E3 are output from the Hall elements 114a, 114b and 114c, respectively. Each of the Hall signals is inverted every 180 degrees with movement of the magnetic poles of the rotor 113 past the corresponding Hall element. The Hall signals are displaced in phase with respect to one another by 60 degrees.
A motor controller 115 comprises a digital signal processor. The motor controller has a sinusoidal amplitude control section 116 for controlling the motor 111 in the steady state after start. The control section, in conjunction with a timing generator 117 provided as an external circuit, controls the amplitude of a sinusoidal signal for keeping the motor at a specified speed on the basis of the Hall signals from the Hall elements 114a to 114c
DA converters 118a, 118b and 118c are connected to the motor controller 115, which convert three-phase sinusoidal amplitude digital data having a phase deviation of 60 degrees output from the control section 116 into analog signals.
The DA converters 118a, 118b and 118c are connected to separation circuits 119a, 119b, and 119c, respectively, which separate sinusoidal signals E16, E17 and E18 from the respective DA converters into drive signals. The driving signals are signals for driving pairs of switching elements of opposite driving polarity provided in a driver circuit 120. The driver circuit is provided with three series circuits, each of a P-channel FET and an N-channel FET, in correspondence with the stator coils 112a, 112b and 112c of the DC motor 111. A switching circuit 121 is equipped with six analog switches 121a to 121f. The switching circuit 121 is connected at its terminals a to the separation circuits 119a to 119c and at its terminals b to a starting controller 122. The switching circuit 121 has its respective switches placed to the b position at the start of the motor by the motor controller 115, applying drive signals from the start controller 122 to the driver circuit 120.
When the motor reaches a specified speed as a result of the start control by the start controller 122, a speed lock signal is obtained from that controller. In response to the speed lock signal, the motor controller 115 moves the switches in the switching circuit 121 to a position. As a result, the drive signals are applied from the separation circuits 119a to 119c to the driver circuit 120.
The start controller 122 is provided with a rectangular-pulse amplitude control section 123, which, in response to the Hall signals E1 to E3 from the Hall elements 114a to 114c, controls the amplitude of rectangular pulses to obtain the specified motor rotation. The amplitude gain of the rectangular pulse signals is controlled so that the difference between the actual rotation detected via the Hall signals and the predetermined reference rotation based on a reference clock becomes zero.
The sinusoidal amplitude control section 116 itself in the motor controller 115 provides no control gain. A drive signal E11 output from the start controller 122 and having its amplitude gain controlled according to the Hall elements 114a to 114c is fed through an AD converter 124 into the motor controller 115. The sinusoidal amplitude control section 116 considers the amplitude of the drive signal E11 to be the control gain for the motor 111 and determines the amplitudes of sinusoidal signals.
Thus, the speed of the brushless DC motor is changed by changing the motor stator currents in analog fashion. Thereby, the motor is allowed to rotate at a target speed.
However, trying to implement the motor speed control by changing the motor stator currents inevitably results in the need of analog circuits. Even if a high-speed digital signal processor is used to detect the difference between specified speed and current speed and to produce digital drive signals, it becomes necessary to change the motor stator coil currents in analog manner in the final stage. Thus, analog circuits, such as DA converters, become necessary. Usually, the motor driver circuit is susceptible to strong noise that arises from the brushless DC motor itself and various components. Analog signals are easy to be distorted by noise. Control of currents in the stator coils of the brushless DC motor by distorted analog signals results in failure to cause desired currents to flow in the stator coils, making the rotation of the DC motor unstable.
Reducing the stator coil currents in the brushless DC motor allows the rotating speed of the motor to be reduced; however, at the same time, the rotation torque will also be reduced. Thus, there arises a problem that the number of rotations of the DC motor varies greatly with varying torque.